In steam power stations, steam is used as working medium for operating steam turbines. Pressurized water vapor is produced in a steam boiler and flows via pipes into the steam turbine. In the steam turbine, the previously absorbed energy of the working medium is converted into kinetic energy. The kinetic energy is used, for example, to operate a generator which converts the mechanical power generated into electrical power. The expanded and cooled steam then flows into a condenser where the steam condenses by heat transfer in a heat exchanger and is fed again as liquid water by a pump to the steam boiler for heating, evaporating and subsequent superheating. In order to achieve greater and maximum efficiency in the steam power process, the steam power process is being developed to ever greater fresh steam parameters. Said high fresh steam parameters displace the condensation point of the system deeper into the wet steam range and therefore to partial condensation.
Customary steam turbine systems have at least one high-pressure part for maximum efficiency. In addition, a medium-pressure part and one or more low-pressure parts can be used. In the low-pressure part, the temperature of the steam drops very abruptly, as a result of which partial condensation of the steam occurs. However, the low-pressure part is highly sensitive, as far as the wet content of the steam is concerned. If the steam in the low-pressure part of the turbine reaches a wet content of approx. 8 to 10 percent, measures have to be taken to reduce the wet content of the steam to a permissible extent prior to entry into the low-pressure part and during the further expansion in same. One of said measures can be the use of an additional resuperheating and/or drying of the steam. By means of this measure, the steam is again resuperheated and therefore the efficiency of the steam power process is increased at the same time.
For the steam drying/resuperheating, the entire mass flow of steam is completely removed from the turbine and fed again to the steam boiler prior to entry into the middle- or low-pressure part. In the resuperheating, the steam temperature is generally raised again to that of the fresh steam, and therefore the wet content at the expansion end point drops. The steam is subsequently guided back into the turbine system. Without such a resuperheating, the steam turbine system cannot be continuously operated at extremely low exhaust steam pressures (approx. 50 . . . 25 mbar) since water droplets which have condensed out strike against the rotating turbine blades and thereby cause damage to the blading.
In the case of multi-casing steam turbine systems, such a resuperheating/drying of the water vapor can be carried out between the individual turbine sections.
The casing material in the inflow region of the turbine is greatly weakened in the strength properties thereof by the very hot steam, and therefore said casing material can no longer counteract the pressures prevailing in the interior. A thickening of the casing wall is possible only to a limited extent since, in the case of very thick casings, impermissibly high, thermally induced stresses occur in the casing wall as a result of temperature changes. The same temperatures prevail in the region of action with the resuperheated steam, and therefore the casing material is also greatly weakened here. Turbine systems with resuperheating therefore differ from conventional systems by means of two points in the expansion run that are at risk because of extremely high temperatures.
In the case of a single-casing steam turbine with resuperheating, greatly superheated steam is conducted into the turbine at two points. The turbine outer casing is greatly loaded thermally at two points by the temperatures and pressures which occur.
Steam turbines with resuperheating have previously been designed either as two-casing turbine systems, or lower steam parameters have been used such that the single-shell turbine outer casing has not been overloaded.
However, the required parameters which occur frequently lie above the possible parameters of single-shell turbine casings.